1. Field of the Invention
This invention relates to an electric double-layer capacitor.
2. Description of the Related Art
An electric double-layer capacitor is a charge storage device which includes: a positive electrode in which a polarizable electrode is closely attached to a positive electrode current collector plate; a negative electrode in which a polarizable electrode is closely attached to a negative electrode current collector plate; a separator; and an electrolytic solution. These positive and negative electrodes face to each other across the separator and the electrostatic capacitance of an electric double layer formed on the polarizable electrodes in the electrolytic solution is utilized. As the major constituent of the polarizable electrodes in electric double-layer capacitors, carbon-based materials having an extremely large surface area per unit mass, such as active carbon, are often used. The electric double-layer capacitor has much larger electrostatic capacitance, as compared with capacitors of, for example, aluminum electrolytic capacitors. Different from charge storage devices with the use of electrochemical reactions such as secondary batteries, charge and discharge of an electric double-layer capacitor are conducted by transferring ions into and from micropores of active carbon. Therefore, the electric double-layer capacitor can be very quickly charged and discharged without resorting to electrochemical reactions. By taking the advantage of these characteristics, it has been considered to employ electric double-layer capacitors for various purposes, for example, a power supply for memory backup system in electronic devices, a charge storage device for home appliances and copy machines, a starting power supply for automotives in idling stop, a power supply for hybrid cars, a power storage system for peak-shaving or leveling in solar photovoltaic generation, and so on. Thus, electric double-layer capacitors are expected as a key device contributing to energy saving and reduction in carbon emission.
In an electric double-layer capacitor, it is an important problem to increase the storage energy which is one of the major performances thereof. Since the storage energy in an electric double-layer capacitor increases in proportion to the square of the charging voltage, elevation in the charging voltage results in an increase in the storage energy. To elevate the charging voltage of an electric double-layer capacitor, it is required to elevate the withstand voltage. Although the withstand voltage of an electric double-layer capacitor varies depending on the electrolytic solution employed, it ranges from about 2.3 to about 2.7 V in a non-aqueous system. To elevate the voltage, there have been proposed a hybrid type electric double-layer capacitor which includes, for example, a combination of a negative electrode having a polarizable electrode containing a carbon material or a metal oxide capable of storing and releasing lithium ion, a positive electrode having a polarizable electrode made of active carbon and an electrolytic solution containing lithium ion (see, for example, Patent Documents 1 to 4). In the case of using lithium titanate in the polarizable electrode of the negative electrode of such a hybrid type electric double-layer capacitor, it is reported the cycle characteristics can be improved since lithium titanate has a redox potential to lithium ion at around 1.5 V and shows little change in volumetric expansion upon the storage and release of lithium ion. As a method of improving the electrical resistance of an electrode, there has been also proposed an electrode in which a small amount (i.e., 1.25% by weight or less based on the active carbon) of a metal oxide and a binder are added to the active carbon so as to protect the active carbon surface from coating with the binder (see, for example, Patent Document 5).    Patent Document 1: JP-A-2002-270175 (page 2)    Patent Document 2: JP-A-2003-132945 (page 2)    Patent Document 3: JP-A-2004-55541 (pages 3 to 4)    Patent Document 4: JP-A-2005-353652 (page 3)    Patent Document 5: JP-A-2001-217162 (pages 2 to 3)
However, such an electric double-layer capacitor with the combined use of an electrolytic solution containing lithium ion with a negative electrode using a carbon material or a metal oxide capable of storing and releasing lithium ion suffers from a problem of having poor cycle characteristics and a low storage stability at high temperature, since an electrochemical reaction arises at the discharge. In an electric double-layer capacitor having a negative electrode made of a material containing lithium titanate, for example, an electrochemical reaction represented by the following formula (1) is employed and, therefore, irreversible degradation of lithium titanate accompanying the discharge is unavoidable, which brings about a problem that the in charge/discharge cycle stability is lowered.Li4Ti5O12+xLi=Li4+xTi5O12  (1)
A hybrid type electric double-layer capacitor having a negative electrode made of a material containing lithium titanate has working voltage of about 2.5V due to a redox potential of lithium titanate, which brings about another problem that the voltage cannot be elevated any more.
When the hybrid type electric double-layer capacitor is held under high-voltage application, there arises a problem that the active carbon surface is coated with decomposition products, which are formed by the decomposition of the electrolytic solution or impurities, or a gas generated by the decomposition is adsorbed by the active carbon surface so that the electrostatic capacitance of the electric double-layer capacitor is lowered and the charge/discharge cycle characteristics are deteriorated.
By using an electrode in which a small amount (i.e., 1.25% by weight or less based on the active carbon) of a metal oxide and a binder are added to the active carbon, it can be expected that the electrostatic capacitance is elevated and the electrical resistance is lowered. In this case, however, there still remains a problem that the charge/discharge cycle characteristics and the storage stability at high temperature cannot be improved. Since an electric double-layer capacitor has a life period of several years, it has been a practice to estimate the life period by accelerating the degradation conducting a float test, wherein a cell is stored under applying voltage at high temperature, as an accelerated life test. It is generally said that the life period is reduced by half with an increase in temperature by 7° C. The term “high voltage” as used herein means voltage of 3.0 V or higher, i.e., exceeding the charge voltage (2.5 V to 2.7 V) of electric double-layer capacitors commonly employed.